Background
There is a continuing need for improved methods to decontaminate various volumes and the objects within them both on a small scale, such as instruments within a chamber and on a large scale, such as entire rooms and the objects within. While there have been attempts to develop methods for decontaminating such enclosures from microbial pathogens, chemical agents, odors, and the like, current methods have shortcomings based on their scalability, the long duration of their operating cycles, the cost associated with the equipment, safety concerns and other limitations. With the continuing spread of resistant bacterial pathogens such as MRSA (methicillin-resistant staph aureus), c.diff (clostridium difficile), viral pathogens such as rotavirus and rhinovirus and other pathogens such as stachybotrys mold, there is a growing need for an effective, economical, rapid method of decontamination. Preferably, in addition to being cost effective, efficacious, and quick, a successful method should be scalable, yet also portable and completely automated to be used in a wide variety of situations with minimal need for reconfiguration.
Hydrogen peroxide and other biocides have been employed in various methods to decontaminate the aforementioned and other contaminants with varying success. While hydrogen peroxide can be an effective decontaminant, challenges exist with deploying it at sufficient concentrations, in sufficient quantities, at a sufficiently rapid rate to be effective, efficient, and adaptable to a wide variety of environments.
Furthermore, the production of condensed vapors of various biocides and neutralizing agents are important for various industrial applications including biodecontamination, sanitation, sterilization, air sterilization, odor removal, factory fumigation, chemical and biological agent neutralization and, use of condensed vapor as a gaseous reagent in gas phase problems.
Brief Discussion of the Prior Art
The prior art teaches various methods for evaporation and vaporization of biocide liquids. These methods include flash vaporization, spray vaporization, and very fine droplet evaporation and drying, among others. Generally, biocide is injected either as a mist, a collection of droplets created through mechanical shearing of the initial liquid, or a vapor, a completely gaseous form of the initial liquid, coming from an apparatus or a device.
Flash Vaporization:
Flash vaporized hydrogen peroxide in biodecontamination processes are taught in U.S. Pat. Nos. 7,014,813; 7,157,046 and 20100143189. In these and many industrial processes, vapors are produced by flash vaporization of solutions or liquid mixtures, or heating of the liquid above its boiling point, on a hot plate or similar massive heat source The vapors are then moved by a carrier gas to discharge them to application areas. Flash vaporization has a number of drawbacks. For example, the vapors are produced above the boiling point and exit with elevated temperatures, the flash vaporization by hot plate and heated blocks is proven to be inefficient in producing large quantity of vapors due to the small surface area per unit of mass of large liquid pools, films, or droplets for heat and mass transfer, and there are severe scaling limitations because of the large-scale heating structures needed to vaporize scalable amount of liquids and thermal inertia problems. The prior art does not teach a methodology of using hydrogen peroxide as a sterilizing agent that would be applicable to the need for an improved method of discharging a condensed vapor or condensed vapor mixture of hydrogen peroxide into the volume to be decontaminated where a concentrated biocide solution might be preferred.
Spray Vaporization and Evaporation:
Another method is spray vaporization and evaporation. U.S. Pat. No. 7,354,551, and U.S. Pat. No. 8,007,717 describe two-phase mixing of spray with hot gas to produce vapor. The vapor is produced by spray vaporization of liquid droplets. The droplets are plunged into a high velocity gas stream or, alternatively, high velocity spray droplets are discharged into a relatively still air environment. The ability to generate a sufficiently high concentration of vapors will depend on the efficiency of evaporation, rather than just the availability of heat or enthalpy for evaporation. The extent and rate of evaporation depends on the droplet size and its size distribution, the number density of droplets, the relative velocities of the air and the droplets, the thermal environment and the humidity conditions. The key to successful evaporation of all the droplets depends on how the droplets interact with the forced convection flow, turbulence, and the rate at which heat and mass is exchanged across the interface.
The most important factor is the surface to volume ratio of droplets followed by its entrainment, turbulence and mixing. Small (less than 10-20 micron) droplets, preferable monodisperse, have a huge surface to volume ratio yielding efficient heat and mass transfer. The local humidity surrounding the individual droplets is also an important factor.
In summary, the prior art methods of producing vapors from liquid systems by spray drying and forced convective evaporation have several shortcomings in an application that requires very rapid and complete evaporation of large quantities of input liquid. The use of sprays to produce the droplets for evaporation involves a wide range of droplet sizes. The larger the droplets, the higher the heat and mass transfer rate requirement in order to completely evaporate the droplets. Thus, the larger droplet sizes associated with the broad droplet size distribution found in most spray systems will not completely evaporate in acceptably short timescales. This renders the system costly and not scalable for large scale vapor production for commercial applications. Furthermore, to solve this problem of larger droplets within the droplet size distribution, spray systems will require increasing amounts of pressure to create finer droplets to avoid long evaporation time scales. This pressure, and the sudden release of this high pressure at the nozzle orifice, can have detrimental effects on various chemical components of biocides to be vaporized and may pose safety concerns. Further, spray nozzles are subject to erosion, clogging and other such problems that can cause inconsistencies in the output droplet sizes which, as is clearly shown above, will have tremendous detrimental impact on the vaporization process. The prior art does not teach a methodology of using hydrogen peroxide as a sterilizing agent that would be applicable to the need for an improved method of creating a monodisperse mist to provide efficient, complete evaporation of a solution in order to subsequently discharge a condensed vapor or condensed vapor mixture of hydrogen peroxide into the volume to be decontaminated where a concentrated biocide solution might be preferred.
Diffusion Mixing Evaporator:
In US patent 2012/125197, a diffusion evaporation process is disclosed. In this process, the fine mist and the hot air mix only beyond the exit opening of the apparatus to the environment; not inside the apparatus. In diffusion evaporation, heat energy in a hot gas passes through an annular space without contacting, pre-heating, or mixing with the mist until they meet beyond the outlet. Just beyond the exit of the apparatus the mist mixes with hot air and subsequently evaporates in the environment. Because the heating of the mist does not occur until the two streams reach the environment outside of the device, the enthalpy associated with the hot air stream dissipates into the environment decreasing the efficiency of the evaporation process considerably. The '197 publication does not include a methodology of using hydrogen peroxide as a sterilizing agent that would be applicable to the need for an improved method of utilizing rapid, enhanced residence time, premixed evaporation of a solution within the device to avoid the loss of efficiency associated with evaporation outside of the device. The '197 publication does not include a methodology to discharge a condensed vapor or condensed vapor mixture of hydrogen peroxide into the volume to be decontaminated where a concentrated biocide solution might be preferred.
Premixed Evaporation Method:
According to the present invention of premixed evaporator method, a fine mist is entrained into a hot air stream at the base of an evaporator tube and the two are premixed before the efficient evaporation takes place. It is beneficial to create a “premixed” gas and mist droplet zone where residence time is long enough and number density is dilute enough to accomplish instantaneous evaporation of the microdroplets. Entrainment of fine droplets into a hot gas stream is the determining factor in the U.S. Pat. No. 7,090,028 disclosures by Adiga et al.
Aerosol containing extremely fine droplets of water, those having diameters of less than 10 microns, behaves like a gas or is a pseudo gas, as further described in K. C. Adiga, Fire & Safety Magazine Spring 2007. The evaporation/drying in a premixed system containing such a fine droplet aerosol and the drying gas inside a dryer tube was reported in U.S. Pat. Nos. 7,724,442 and 7,744,786. These patents teach the premixing of a slow moving ultrafine mist of water containing a dissolved solute premixed with a drying gas, such as hot air, with a high enough residence time due to swirling flow. A “premixed” evaporator was disclosed for producing nanoparticles through drying and rejecting the solvent vapor. Adiga describes the drying and evaporation of liquids at low temperature using a swirl flow tube. Although it contains solute additives, the major phenomenon is the efficient evaporation of liquid within the tube. Applications requiring a fast and complete evaporation of droplets require an efficient way of supplying both the proper thermal environment and adequate mixing of the extremely fine droplets within the evaporation volume. While forced convection helps the rate via enhanced heat and mass transfer across the surface, this alone does not guarantee the droplet entrainment into the air, mixing by turbulence and subsequent heat and mass transfer processes. Beyond these, the limiting factor is the number density of droplets that controls the local humidity around individual droplets. At high enough local relative humidity immediately adjacent to the droplet, the evaporation rate falls. To improve the evaporation process will require high temperatures to raise the saturation level of the locally humid air. Any discussion of humidity in this disclosure is purely related to the function of drying the solid and had no bearing on the discharged vapor or its use. In this disclosure, Adiga ignores the vapor and focuses on the dried particles which are collected as manufactured materials. While the disclosed invention does address the heat and mass transfer of evaporation, Adiga does not include a methodology of using a multicomponent liquid-liquid solution without a solid solute included, since this would have no value in creating particles.
A method is needed to provide an efficient mist evaporation process and device producing relatively low temperature vapors which can condense and form concentrated condensed vapor upon coming in contact with cooler air with a potential for seamless scaling capability. Providing a premixed evaporator is an industrially important concept for generating large quantity of completely vaporized premixed mist and warm/hot air system, which can be discharged as condensed vapor cloud, vapor or a mixture.